James 2:11

εἰ δὲ οὐ μοιχεύεις. This use of οὐ for μή in the protasis of a conditional sentence is not infrequent in the N.T., especially when there is an antithesis between a negative and affirmative sentence as here. As Dr Moulton notes on Winer, p. 601, the expression is equivalent to εἰ οὐ μοιχεύων ἔσῃ, φονεύων δέ. εἰ οὐ μοιχεύεις = ‘if thou art guiltless of adultery.’ Comp. εἰ τοὺς θανόντας οὐκ ἐᾷς θάπτειν, Soph. Aj. 1131; εἰ�, Thuc. III. 55; εἰ ἄλλοις οὐκ εἰμὶ�, 1 Corinthians 9:2.

In Modern Greek ἐὰν δέν (the modern equivalent to οὐ) is the regular idiom.